1. Field of the Invention
The present invention relates to a power amplification circuit for a communication device, and more particularly, to a power amplification circuit for a communication device suitable for use in mobile communication.
2. Description of the Related Art
In the field of mobile communication in recent years, there is a sharp increase in traffic because portable terminals have been widely used and therefore a plurality of communication systems exist whose frequency bands are largely apart from each other.
In order to improve users' convenience, it is demanded by the market that these plurality of communication systems should be realized by one terminal device.
Under these circumstances, for the transmission and reception of signals at a plurality of communication systems, it is desirable in terms of costs and mounting areas, rather than that one terminal device is mounted with a plurality of circuits for transmitting and receiving signals whose frequencies for use are largely apart from each other as many as the number of communication systems, that a part of the circuit is shared. This is obvious because the market demand trends toward terminal down-sizing.
Presented as a conventional example of a power amplification circuit for a communication device capable of transmitting signals of a plurality of frequency bands is an example of a power amplification circuit structure shown in FIG. 4 or 5.
A power amplification circuit 400 shown in FIG. 4 is structured such that an initial-stage amplifier 401 for amplifying power of a modulation signal output from a signal source 470 is used in common by two frequency bands and at the subsequent stages, a transmission signal of one frequency band has its power amplified by amplifiers 402 and 403 and a transmission signal of the other frequency band has its power amplified by amplifiers 404 and 405 and the amplified signals are output as external signals.
On the other hand, a conventional power amplification circuit 500 shown in FIG. 5 is structured such that two amplification systems completely independent of each other, that is, an amplification system composed of amplifiers 501, 502 and 503 for amplifying an output signal of a signal source 580 and an amplification system composed of amplifiers 511, 512 and 513 for amplifying an output signal of a signal source 590, respectively amplify power of the output signals of the signal sources 580 and 590 which output signals whose frequency bands are apart from each other and respectively output these amplified outputs as independent transmission signals.
Although the conventional power amplification circuit shown in FIG. 4 has a less number of components than that of a power amplification circuit according to an embodiment of the present invention which will be described later, when two frequency bands are apart from each other as described above, it is extremely difficult with respect to the two frequency bands to ensure high efficiency of power amplification at the initial-stage amplifier 401, whereby frequency characteristics (amplification characteristics) of one of the frequency band is largely limited.
The conventional power amplification circuit shown in FIG. 5, unlike the power amplification circuit shown in FIG. 4, enables optimization of the frequency characteristics of the initial-stage amplifiers 501 and 511 in the respective frequency bands.
However, as much space for a semiconductor die of the initial-stage amplifier and a matching circuit and the like to be formed preceding thereto is as a matter of course needed as a space equivalent to the number of frequency bands. This is not desirable in terms of costs and size.
As described in the foregoing, a power amplification circuit for use in a conventional communication device, in a case where the communication device has a plurality of transmission frequency bands, is provided with a plurality of the same circuits even though a circuit size is increased or has the circuit partly shared while sacrificing amplification efficiency in a certain frequency band.